It's created when the data is written to both drives.

This is standard stuff, very well proven: try googling 'self healing zfs'.

- Luke

Msg is shrt cuz m on ma treo

-----Original Message-----
From: Michael Stone [mailto:mstone+postgres@mathom.us]
Sent: Wednesday, May 30, 2007 11:11 AM Eastern Standard Time
To: pgsql-performance@postgresql.org
Subject: Re: [PERFORM] setting up raid10 with more than 4 drives

On Wed, May 30, 2007 at 10:36:48AM -0400, Luke Lonergan wrote:
>> I don't see how that's better at all; in fact, it reduces to
>> exactly the same problem: given two pieces of data which
>> disagree, which is right?
>
>The one that matches the checksum.

And you know the checksum is good, how?

Mike Stone

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> This is standard stuff, very well proven: try googling 'self healing zfs'.

The first hit on this search is a demo of ZFS detecting corruption of one of
the mirror pair using checksums, very cool:

http://www.opensolaris.org/os/commun...essionid=52508
D464883F194061E341F58F4E7E1

The bad drive is pointed out directly using the checksum and the data
integrity is preserved.

- Luke

On Wed, May 30, 2007 at 08:51:45AM -0700, Luke Lonergan wrote:
> > This is standard stuff, very well proven: try googling 'self healing zfs'.
> The first hit on this search is a demo of ZFS detecting corruption of one of
> the mirror pair using checksums, very cool:
>
> http://www.opensolaris.org/os/commun...essionid=52508
> D464883F194061E341F58F4E7E1
>
> The bad drive is pointed out directly using the checksum and the data
> integrity is preserved.

One part is corruption. Another is ordering and consistency. ZFS represents
both RAID-style storage *and* journal-style file system. I imagine consistency
and ordering is handled through journalling.

Cheers,
mark

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Sorry for posting and disappearing.

i am still not clear what is the best way of throwing in more
disks into the system.
does more stripes means more performance (mostly) ?
also is there any thumb rule about best stripe size ? (8k,16k,32k...)

regds
mallah



On 5/30/07, mark@mark.mielke.cc <mark@mark.mielke.cc> wrote:
> On Wed, May 30, 2007 at 08:51:45AM -0700, Luke Lonergan wrote:
> > > This is standard stuff, very well proven: try googling 'self healing zfs'.
> > The first hit on this search is a demo of ZFS detecting corruption of one of
> > the mirror pair using checksums, very cool:
> >
> > http://www.opensolaris.org/os/commun...essionid=52508
> > D464883F194061E341F58F4E7E1
> >
> > The bad drive is pointed out directly using the checksum and the data
> > integrity is preserved.
>
> One part is corruption. Another is ordering and consistency. ZFS represents
> both RAID-style storage *and* journal-style file system. I imagine consistency
> and ordering is handled through journalling.
>
> Cheers,
> mark
>
> --
> mark@mielke.cc / markm@ncf.ca / markm@nortel.com __________________________
> . . _ ._ . . .__ . . ._. .__ . . . .__ | Neighbourhood Coder
> |\/| |_| |_| |/ |_ |\/| | |_ | |/ |_ |
> | | | | | \ | \ |__ . | | .|. |__ |__ | \ |__ | Ottawa, Ontario, Canada
>
> One ring to rule them all, one ring to find them, one ring to bring them all
> and in the darkness bind them...
>
> http://mark.mielke.cc/
>
>
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Mark,

On 5/30/07 8:57 AM, "mark@mark.mielke.cc" <mark@mark.mielke.cc> wrote:

> One part is corruption. Another is ordering and consistency. ZFS represents
> both RAID-style storage *and* journal-style file system. I imagine consistency
> and ordering is handled through journalling.

Yep and versioning, which answers PFC's scenario.

Short answer: ZFS has a very reliable model that uses checksumming and
journaling along with block versioning to implement "self healing". There
are others that do some similar things with checksumming on the SAN HW and
cooperation with the filesystem.

- Luke



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On Thu, May 31, 2007 at 01:28:58AM +0530, Rajesh Kumar Mallah wrote:
> i am still not clear what is the best way of throwing in more
> disks into the system.
> does more stripes means more performance (mostly) ?
> also is there any thumb rule about best stripe size ? (8k,16k,32k...)

It isn't that simple. RAID1 should theoretically give you the best read
performance. If all you care about is read, then "best performance" would
be to add more mirrors to your array.

For write performance, RAID0 is the best. I think this is what you mean
by "more stripes".

This is where RAID 1+0/0+1 come in. To reconcile the above. Your question
seems to be: I have a RAID 1+0/0+1 system. Should I add disks onto the 0
part of the array? Or the 1 part of the array?

My conclusion to you would be: Both, unless you are certain that you load
is scaled heavily towards read, in which case the 1, or if scaled heavily
towards write, then 0.

Then comes the other factors. Do you want redundancy? Then you want 1.
Do you want capacity? Then you want 0.

There is no single answer for most people.

For me, stripe size is the last decision to make, and may be heavily
sensitive to load patterns. This suggests a trial and error / benchmarking
requirement to determine the optimal stripe size for your use.

Cheers,
mark

--
mark@mielke.cc / markm@ncf.ca / markm@nortel.com __________________________
.. . _ ._ . . .__ . . ._. .__ . . . .__ | Neighbourhood Coder
|\/| |_| |_| |/ |_ |\/| | |_ | |/ |_ |
| | | | | \ | \ |__ . | | .|. |__ |__ | \ |__ | Ottawa, Ontario, Canada

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On 5/31/07, mark@mark.mielke.cc <mark@mark.mielke.cc> wrote:
> On Thu, May 31, 2007 at 01:28:58AM +0530, Rajesh Kumar Mallah wrote:
> > i am still not clear what is the best way of throwing in more
> > disks into the system.
> > does more stripes means more performance (mostly) ?
> > also is there any thumb rule about best stripe size ? (8k,16k,32k...)
>
> It isn't that simple. RAID1 should theoretically give you the best read
> performance. If all you care about is read, then "best performance" would
> be to add more mirrors to your array.
>
> For write performance, RAID0 is the best. I think this is what you mean
> by "more stripes".
>
> This is where RAID 1+0/0+1 come in. To reconcile the above. Your question
> seems to be: I have a RAID 1+0/0+1 system. Should I add disks onto the 0
> part of the array? Or the 1 part of the array?

> My conclusion to you would be: Both, unless you are certain that you load
> is scaled heavily towards read, in which case the 1, or if scaled heavily
> towards write, then 0.

thanks . this answers to my query. all the time i was thinking of 1+0
only failing to observe the 0+1 part in it.

>
> Then comes the other factors. Do you want redundancy? Then you want 1.
> Do you want capacity? Then you want 0.

Ok.

>
> There is no single answer for most people.
>
> For me, stripe size is the last decision to make, and may be heavily
> sensitive to load patterns. This suggests a trial and error / benchmarking
> requirement to determine the optimal stripe size for your use.

thanks.
mallah.

>
> Cheers,
> mark
>
> --
> mark@mielke.cc / markm@ncf.ca / markm@nortel.com __________________________
> . . _ ._ . . .__ . . ._. .__ . . . .__ | Neighbourhood Coder
> |\/| |_| |_| |/ |_ |\/| | |_ | |/ |_ |
> | | | | | \ | \ |__ . | | .|. |__ |__ | \ |__ | Ottawa, Ontario, Canada
>
> One ring to rule them all, one ring to find them, one ring to bring them all
> and in the darkness bind them...
>
> http://mark.mielke.cc/
>
>

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